A light-emitting diode (LED) is an element that emits light when a forward current passes through the pn junction of a semiconductor, and is manufactured using a Group III-V semiconductor crystal such as GaAs or GaN. In recent years, through advances in epitaxial growth technology for semiconductors and light-emitting element processing technology, LEDs with excellent conversion efficiency have been developed and widely used in various fields.
An LED is composed of a p-type layer and an n-type layer formed by epitaxially growing a Group III-V semiconductor crystal on a monocrystalline growth substrate, and a photoactive layer sandwiched between the two. In general, they are formed by epitaxially growing a Group III-V semiconductor crystal on a growth substrate such as a monocrystalline sapphire, then attaching electrodes thereto (Patent Document 1).
In cases where a Group III-V semiconductor crystal is epitaxially grown on a monocrystalline growth substrate, since the monocrystalline growth substrate and Group III-V semiconductor crystal have different lattice constants, it is difficult to grow a good LED. For this reason, a method of forming a buffer layer such as GaN on a monocrystalline growth substrate at a low temperature, and then epitaxially growing GaN thereon has been proposed (Patent Document 2).
On the other hand, there is the problem of poor thermal conductivity of monocrystalline growth substrates. In the case of monocrystalline sapphire, the thermal conductivity is approximately 40 W/mK, and the heat generated by a Group III-V semiconductor element cannot be sufficiently dissipated. In particular, for high-output LEDs where a large electrical current passes through, there are problems of increased temperature of the elements, reduced luminescent efficiency and reduced lifespan of the elements. For this reason, a method of epitaxially growing a Group III-V semiconductor crystal on a monocrystalline growth substrate, then joining it with a highly thermal conductive substrate via a metal layer, and then removing the monocrystalline growth substrate has been proposed (Patent Document 3). In this case, as the highly thermal conductive substrate, materials such as copper that have excellent thermal conductivity have been examined, but the difference in thermal expansion coefficient with Group III-V semiconductor crystals is large, and their use in high-output LEDs has not been sufficiently satisfactory.    Patent Document 1: JP-A 2005-117006    Patent Document 2: JP-B H5-73252    Patent Document 3: JP-A 2006-128710